The present invention relates to a system for purifying exhaust gas emitted from an engine, which includes a HC trap catalyst for trapping hydrocarbons, referred to hereinafter as HC, and specifically, relates to a technology of allowing for rising a temperature of the exhaust gas and promoting activation of the HC trap catalyst using heat quantity of the exhaust gas.
An exhaust gas purifying system for engines, which includes a HC trap catalyst is well known. The HC trap catalyst is constructed to trap HC contained in exhaust gas emitted from the engine when the HC trap catalyst is in a low temperature condition. Further, when temperature of the exhaust gas rises, the HC trap catalyst purifies nitrogen oxides, referred to hereinafter as NOx, which are contained in the exhaust gas, by the effect of HC trapped by the HC trap catalyst as a reducing agent. Here, if the HC trap catalyst traps sulfur components more than a given amount thereof, the HC trap catalyst will be deteriorated in capability. The HC trap catalyst, therefore, must be periodically heated to remove the sulfur components trapped therein.
Japanese Patent Application First Publication No. 2000-018024 shows an emission control for diesel engines, which uses a light oil adding device. When an exhaust gas purifying catalyst is heated for removing the sulfur components trapped, an excess air ratio is adjusted in the range of 1 to 1.5 by controlling an intake throttle valve and an exhaust gas recirculation valve. Further, the light oil adding device is actuated to mix light oil into exhaust gas, depending on a remaining oxygen content in exhaust gas. The exhaust gas mixed with light oil is introduced into the catalyst in which the light oil in the exhaust gas is oxidized. The catalyst is heated using heat generated upon the oxidation of the light oil.